Project Summary Sarcoidosis is a systemic granulomatous disorder of unknown etiology which affects the lung in greater than 90% of cases. The disease is characterized by the accumulation of activated CD4+ T cells in the lung and other sites of disease activity. Evidence suggests that these T cells are intimately involved in the pathogenesis of sarcoidosis. In an acute form of sarcoidosis known as Lfgren?s syndrome, a correlation between the presence of the HLA-DRB1*03:01 allele and expansions of lung CD4+ T cells expressing the T cell receptor (TCR) ?- chain variable (V) region V?2.3 has been observed. These T cell populations are oligoclonal in nature, suggesting their recruitment to the lung in response to conventional antigen. In addition, these T cell expansions are compartmentalized to the lung and disappear with disease remission, underscoring their importance in disease pathogenesis. We hypothesize that in HLA-DRB1*03:01 individuals with Lfgren?s syndrome, V?2.3-expressing CD4+ T cell clones accumulate and expand in the lungs in response to the etiologic sarcoidosis antigen and recognize that antigen in an HLA-DRB1*03:01-restricted fashion. This proposal harnesses the strengths of an international research team and focuses on a distinct cohort of HLA- DRB1*03:01+ Swedish subjects with Lfgren?s syndrome in which two of the three components of the trimolecular complex are known (i.e., HLA-DRB1*03:01 and V?2.3-expressing CD4+ T cells). Using a single cell RT-PCR approach, Aim 1 will characterize the ??TCR pairs expressed on CD4+ V?2.3-expressing T cells in the BAL of Lfgren?s syndrome patients and generate hybridomas expressing the TCRs of interest. The second specific aim will determine the peptides that stimulate the CD4+ T cell hybridomas expressing disease- relevant TCRs derived from Lfgren?s syndrome patients. The final aim will use HLA-DR3-peptide tetramers to identify antigen-specific CD4+ T cells in the lungs of sarcoidosis patients and determine if the frequency of these T cells can serve as a biomarker for diagnosis and prognosis. Thus, using innovative and novel technology, we will address critical knowledge gaps in the etiologic T cell antigens involved in the initiation of disease in patients with Lfgren?s syndrome, further advancing our understanding of the pathogenesis of this disease. In addition, these results will lay the foundation for additional studies of sarcoidosis in the US.